The amino-oligopeptidases of rat small intestinal brush border are known to play a crucial role in assimilation of dietary protein. They will be thoroughly characterized structurally and their capacity to digest dipeptides with bulky side-chains will be defined. Regulation of the enzyme will be studied by use of monospecific antiserum from rabbits to isolate enzyme after pulse-labeling with radioactive amino acids and by identification of any precursor fragments reacting with the antiserum. The topography of these enzymes in the surface membrane will be determined by probing the luminal intramembrane, and interior surface of the membrane with radioactive covalent ligands and isolating the aminopeptidases, their subunits and fragments. The comparative structural characteristics of the putative aminopeptidase precursor in endoplasmic reticulum and golgi membranes will be compared with those for the mature enzyme in the brush border. Finally the regulatory role of luminal nutrients such as peptides and amino acids on the intracellular synthesis and assembly of the aminopeptidase will be studied in vivo.